Like many glaciers in this part of the Antarctic, the Marr has been retreating at a surprising rate over the past quarter century. Every year, more rock emerges from beneath the melting wall of ice behind Palmer Station, a 40-man United States research station run by the National Science Foundation. Scientists arriving after a year's absence are surprised to find new beaches, outcroppings, even islands that had been hidden for thousands of years under the ice.

"When I was a student, I remember that climate change was envisioned as a slow process that could not be observed in a human lifetime. But indeed, that's very wrong," says William Fraser of Montana State University who, has been coming here since 1974. "Climate change can be a very rapid event, with abrupt and dramatic consequences for the landscape and ecology."

The Antarctic Peninsula region - a thousand-mile arm of glaciated land and islands reaching northward toward the tip of South America - has seen average temperatures increase by almost 5 degrees F. in the last 50 years.

As the region warms, glaciers have retreated, and floating ice shelves that may have formed many thousands of years ago have collapsed. Glaciologists ponder the possible future ramifications for even larger ice formations to the south. Collapse would raise world sea levels by as much as 18 feet.

Rodolfo del Valle, director of geoscience at the Argentine Antarctic Institute in Buenos Aires, knows just how dramatic climate change can be. In 1995, he and his colleagues were at their base camp on a rocky outcropping in the midst of the Larsen-A Ice Shelf, a floating ice sheet the size of Rhode Island and 500 feet thick. They used the shelf as a sort of highway, driving snow machines between geological sites on the peninsula.

One day, the shelf collapsed with a thunderous roar. In a mere two hours, the Argentine team found themselves standing not on a rocky outcropping, but on an island surrounded by open water and enormous icebergs.

"I felt a sadness, a pain in my heart for the loss of a place that had become like a home to me," del Valle recalls. "I've experienced strong earthquakes on land, but this was different. After an earthquake something remains. But not with the ice shelf - it was completely destroyed."

The much smaller Wordie ice shelf disappeared in the late 1980s. Cracks have formed in the Larsen-B shelf - now the northernmost shelf on the continent - and experts believe it could break up at any time.

Because they are already floating, the collapse of these ice shelves does not affect global sea levels. But for decades, scientists have worried that the warming of the peninsula could cause the catastrophic breakup of an enormous, unstable ice sheet at the peninsula's base. This West Antarctic Ice Sheet (WAIS) is the size of Mexico and contains enough water to raise world sea levels by 18 feet.

Fortunately, scientists are reasonably certain that WAIS won't suffer a catastrophic collapse anytime soon. Temperatures would probably have to increase another 10 degrees to trigger the melting of the floating ice shelves at WAIS's base. "Even if those shelves were to disintegrate, we don't know how the grounded ice sheet would respond," says glaciologist Robert Bindschadler of the Goddard Space Flight Center in Greenbelt, Md.

If temperatures continue to rise - and climate models suggest they will - WAIS could start shedding ice at an increased rate. "We don't think there will be a catastrophic collapse in the next century, but what about a 5 or 10 percent loss?" says David Vaughan of the British Antarctic Survey. "Just a 5 percent contribution from WAIS could double the predicted rise in sea levels."

The Intergovernmental Panel on Climate Change - the group of scientists responsible for predicting likely effects of climate change - assumes the Antarctic will have a zero net contribution to sea-level rise in the next century. Increased snow fall, they reason, will balance out meltwater from retreating glaciers. They also assume that WAIS will remain stable.